1. Field of the Invention
The present invention relates to the field of silica crucibles and more particularly molds in which such crucibles are made and methods for their manufacture.
2. Description of the Related Art
Silicon wafers used in the semiconductor industry are made from ingots of single crystalline silicon. Such ingots are generally manufactured using the Czochralski (CZ) process. In the CZ process, metallic silicon is charged in a silica glass crucible housed within a susceptor that is received in a crystal growth chamber. A heater surrounding the susceptor heats the charge thus melting the silicon. A single silicon crystal is then pulled from the melt at or near the melting temperature of silicon.
The crucibles used for this process have rounded bottoms and a cylindrical wall, which are supported by the susceptor during the CZ process. They are made using rotating graphite molds that have air channels communicating with the interior of the mold at the sides and bottom thereof. While the mold rotates, high quality silica grain is distributed over the surface of the mold and shaped in a known manner. Electrodes are then lowered into the mold, and power in the range of 300 KVA to 1200 KVA is applied, thus creating ball of plasma gas. The heat so generated fuses the silica in the shape of a crucible suitable for use with the CZ process.
The graphite mold, or insert, used to make such crucibles is received in a metal container known in the industry as a can. The insert is cylindrical in shape with an outer cylindrical wall sized to be received in the can. The graphite insert includes a mold cavity having a rounded lower portion for shaping the lower end of the crucible and a cylindrically shaped inner wall surface for shaping the upright crucible wall. The air channels mentioned above communicate with the surface of the mold cavity.
Creation of the graphite insert is complex and expensive. It is made from slurry that is purified, heat treated, and shaped into a cylindrical blank in an isostatic press. For some crucibles, the blank must have a 36-inch diameter. The height required for some of the larger graphite molds is limited by the capabilities of the press. The graphite blank so formed is then machined to shape the cavity in which the crucible is formed as well as the air channels.
On one hand, graphite is a desirable substance to use as a crucible mold because it can be relatively easily shaped in the form necessary to mold the crucible. And it is better suited than some materials to withstand the heat generated by the plasma gas and the silica during the fusion process. On the other hand, it wears more quickly than metal and so becomes worn and must be periodically replaced. Metal, though, is very difficult to shape in the form needed to mold the crucible. The best approach known to date is to use the easily machined graphite while accepting that it must be frequently replaced due to its high rate of wear.
Another drawback of graphite compared to metal is that graphite takes longer to cool down than metal. This slows down crucible throughput and increases cost of production. Because there is a space between the exterior portion of the graphite mold and the can, air in that space acts as an insulator, which retains heat, i.e., slows cooling after fusion is complete.
Finally, fusion of the crucible proceeds from the radially innermost layer of silica to the radially outermost layer. Before the innermost layer fuses, gas is drawn through the silica and into the air channels in the graphite mold by a vacuum pump. Once the innermost layer fuses, the vacuum draws air only through the uppermost portion of the shaped silica about its periphery. This creates a strong flow of air in the silica grain layer between the fused crucible surface and the surface of the mold cavity. The flow is from the very top of the silica and into the air channels formed in the side and bottom of the mold.
While this flow is needed to remove gasses and prevent bubbles as described above, it also creates a lot of wear to the graphite mold. Such wear occurs around the periphery at the top. In addition, channels form on the mold surface as a result of gas flow down to the air channel bores in the mold. These channels form vertical grooves that extend upwardly from the bores formed on the side wall of the graphite mold.